Production of Polymer Beads
The production of polymer beads by controlled droplet coalescence in a laminar liquid stream followed by dispersion polymerization in a descending column of liquid to produce hardened beads which are subsequently further heated for 6-8 hours to complete polymerization is disclosed in U.S. Pat. No. 4,424,318 (Vairetti et al.). The beads are a copolymer of styrene and divinylbenzene, and the dispersion medium is aqueous calcium chloride containing bentonite and sodium lignosulphonate. The process is inapplicable to rapidly polymerizing mixtures owing to the prolonged coalescence stage, there is no disclosure or suggestion that polymerization can be completed in a laminar flow column, nor is there any suggestion that the dispersion medium can be other than aqueous. The knowledge and belief of the inventors is that the Vairetti process has not been commercialized and that production of styrene/divinylbenzene beads remains a stirred batch process.
Preparation of beads of inter alia styrene/divinylbenzene copolymer is also disclosed in U.S. Pat. No. 6,492,471 (Eisenbeiss et al., Merck) where it is explained that the problem of producing bead polymers on a large industrial scale remains unsolved. The reasons include mixing problems, problems in obtaining a desired bead size distribution and the formulation of caked aggregates in dead spaces. The disclosed solution is to use high performance micromixers. The continuous phase is water, water/alcohol or water/organic solvent and the dispersed phase is a hydrocarbon or sparingly water-miscible hydrocarbon which contains the monomer or monomers to be polymerized. Particular preference is given to the so-called LIGA micromixing system (micromixer) from IMM (Institut fur Mikrotechnik Mainz GmbH) in which two liquid streams are transported in opposite directions to one another in specially shaped microchannels in a mixing unit and subsequently discharged through a slot perpendicular to the flow direction. Production of particles of size 0.1-300 μm is envisaged, more typically 1-20 μm or 10-50 μm, and the formation of larger droplets is not disclosed. The use of micromixers is inappropriate for polymerizable liquid precursors whose rate of polymerization is relatively rapid at the intended mixing temperature because in such mixers the size of the channels is the same as that of the beads produced and there is a high probability of the mixer being blocked by accumulating deposits of polymer. Furthermore, the use of laminar flow conditions for polymerization of the dispersed droplets is neither disclosed nor suggested.
A problem with which this invention is concerned is to provide a process which can produce resin beads on an industrial scale without aggregates of resin building up speedily and interrupting production.
Production of Mesoporous Beads
That problem has become relevant to the present applicants who are seeking to produce on an industrial scale mesoporous beads of phenolic resin as intermediates in the production of beads of carbon having a mesoporous/microporous pore structure.
WO 02/12380 (Tennison et al., the disclosure of which is incorporated herein by reference) discloses making a mesoporous resin by condensing a nucleophilic component which comprises a phenolic compound or a phenol condensation prepolymer with at least one electrophilic cross-linking agent selected from formaldehyde, paraformaldehyde, furfural and hexamethylene tetramine in the presence of a pore-former selected from the group consisting of a diol (e.g. ethylene glycol), a diol ether, a cyclic ester, a substituted cyclic ester, a substituted linear amide, a substituted cyclic amide, an amino alcohol and a mixture of any of the above with water to form a resin. The pore-former is present in an amount effective to impart mesoporosity to the resin (e.g. at least 120 parts by weight of the pore former being used to dissolve 100 parts by weight of the nucleophilic component, which corresponds to 100 parts by weight of the total resin forming components, i.e. nucleophilci component plus electrophilic component), and it is removed from the porous resin after condensation by washing or by vacuum drying. The resulting resin may be carbonised by heating in an inert atmosphere to a temperature of at least 600° C. to give a material having a bimodal distribution of pores, the pore structure as estimated by nitrogen adsorption porosimetry comprising micropores of diameter up to 20 Å and mesopores of diameter 20-500 Å, and the material also containing macropores. The value for the differential of pore volume with respect to the logarithm of pore radius (dV/d log R) for the mesopores is greater than 0.2 for at least some values of pore size in the range 20-500 Å The mesoporous carbon may have a BET surface area of 250-800 m2/g without activation. It may be activated by heating it at high temperature in the presence of carbon dioxide, steam or a mixture thereof, e.g. by heating it in carbon dioxide at above 800° C., or it may be activated by heating it in air at above 400° C. It may then have surface areas of up to 2000 m2/g. As used herein the term “BET surface area” is determined by the Brunauer, Emmett, and Teller (BET) method according to ASTM D1993-91, see also ASTM D6556-04.
In WO 02/12380, production of the resin in both powder and bead form is disclosed. Production of the bead form may be by pouring partially cross-linked pre-polymer into a hot liquid such as mineral oil containing a dispersing agent and stirring the mixture. The pre-polymer solution forms into beads which are initially liquid and then, as curing proceeds, become solid. The average bead particle size is controlled by several process parameters including the stirrer type and speed, the oil temperature and viscosity, the pre-polymer solution viscosity and volume ratio of the solution to the oil and the mean size can be adjusted between 5 and 2000 μm, although in practice the larger bead sizes are difficult to achieve owing to problems with the beads in the stirred dispersion vessel. The beads can then be filtered off from the oil. In a preparative example, industrial novolac resin is mixed with ethylene glycol at an elevated temperature, mixed with hexamine and heated to give a viscous solution which is poured into mineral oil containing a drying oil, after which the mixture is further heated to effect curing. On completion of curing, the reaction mixture is cooled, after which the resulting mesoporous resin is filtered off, and washed with water to remove pore former and a small amount of low molecular weight polymer. The cured beads are carbonized to mesoporous carbon beads which have a pore structure as indicated above, and may be activated as indicated above. It is stated that the beads can be produced with a narrow particle size distribution e.g. with a D90.D10 of better than 10 and preferably better than 5. However, the bead size distribution that can be achieved in practice in stirred tank reactors is relatively poor, and the more the process is scaled up the worse the homogeneity of the mixing regime and hence the particle size distribution becomes.
Patent Application WO 2006/103404 (Cashmore et al., British American Tobacco Co, the disclosure of which is incorporated herein by reference) discloses that the above mesoporous carbon beads are suitable for incorporation into smoke filters of cigarettes, and that carbonized resins obtained from phenoloc resins cross-linked by nitrogen-containing cross-linking agents e.g. hexamethylenetetramine or melamine or produced from nucleophilic precursors such as amino phenols are particularly effective for removing hydrogen cyanide from the vapour phase of tobacco smoke, and also perform well in the removal of formaldehyde, acetaldehyde and 1,3-butadiene. Porous carbon in the form of microbeads e.g. of size 50-1000 μm is said to be particularly suitable for handling in the manufacture of smoking articles because the microbeads have a reduced risk of sticking together and giving rise to uneven loading of absorbent material tow for forming into cigarette filters, and because they have a low attrition rate and therefore generate less dust as compared to known forms of carbon e.g. coconut charcoal.
A more specific problem with which the invention is concerned is therefore the production of mesoporous beads of phenolic resin on an industrial scale without rapid formation of aggregates of polymerized material that would interfere with production.
Bead Carbonization and Activation
Both carbonization and activation of carbonaceous materials in rotary kilns is known, but the processes involved in practical production are slow and the materials produced vary in their properties.
U.S. Pat. No. 1,505,517 (Woodruff et al.) discloses the activation of carbon in a rotary kiln rotating at about 1 revolution every 2 minutes, inclined at a small angle to the horizontal and provided with flights which serve both to agitate the material within the kiln and to elevate that material and drop it through the kiln atmosphere, the preferred activating material being steam, although the use of carbon dioxide is also mentioned. Treatment of highly flowable materials in bead form is not disclosed and no means is provided for retarding the flow of material through the furnace and hence of controlling residence time.
U.S. Pat. No. 4,344,821 (Angelo) discloses a process for simultaneous drying, carbonization and activation of carbonaceous material of animal or vegetable origin in a rotary kiln. It is explained that once the carbonization reaction is initiated, it is self-sustaining, but that that the heat generated is insufficient to dry the incoming material. Air is introduced into the kiln to partially combust the gases given off during carbonization and hence provide the heat needed for the drying stage. The present inventors are of the view that any introduction of air into the kiln is to be avoided, especially in the case of mesoporous materials. For activation, superheated steam is injected at the lower end region of the kiln directly into the bed of char for the purpose of activating the char without steem circulating into the region above the bed. Again, treatment of highly flowable materials in bead form is not disclosed and no means is provided for retarding the flow of material through the furnace and hence of controlling residence time.
U.S. Pat. No. 6,316,378 (Gibelhausen et al., CarboTex GmbH) is concerned with the production of carbon beads from resinous raw materials e.g. ion exchange beads using a rotary tunnel dryer. In an example, resin was supplied to a rotary drying kiln having a length to diameter ratio of 5.5 to achieve a filling level of 20%, the kiln having lift scoops for turning over the product and the product having a transport speed of 11.1 cm/minute, drying being in a countercurrent of hot gas. Carbonization and activation were then carried out in a rotary tunnel kiln filled to a filling ratio of 11%, having a co-current flow of steam and employing temperatures of 850-900° C. The kiln had a length to diameter ratio of 12, the transport speed of the product was 28 cm/minute, the residence time was 514 minutes, and the steam was introduced about 20% of the distance along the kiln. The disclosed drying conditions would destroy mesoporosity in the resin beads. According to the calculations of the present inventors, the dryer length was 5 meters, the pyrolysis region was 36 meters long and the activation region was about 144 meters long. Although the kiln sloped downwardly, it must have been at a very shallow angle owing to its length, and there is no disclosure or suggestion of annular weirs to control the flow of beads along the furnace and hence the residence time.